The present invention pertains to a system for transmitting acoustic data through a tubing string.
There has been much interest in transmitting acoustic signals to and from locations in an oil well environment. The basic operating principal in acoustic signal transmission in a tubular media is to impart propagating stress waves into a pipe or tubing string which travel within the pipe to a distant location where transducers detect the signal which is then interpreted by the receiving equipment. In this way, data and signals can be transmitted via mechanical tubular transmission channels such as pipe or tubing.
There are many practical problems associated with using this scheme. When tubing, drill pipe or casing are used as an acoustic transmission channel, there is often significant signal distortion due to reflective interfaces in the channel such as tool joints, collars or other upsets. Additionally, there can be significant attenuation and interference associated with the fluid system within the wellbore and echos of the acoustic signals themselves within the wellbore. These factors significantly reduce the conditions under which acoustic data transmission may be effectively utilized. Acoustic data transmission may be limited by the distance of the transmission, the number and type of upsets in a drill string.
Efforts to effectively transmit data acoustically have often centered on careful control of the frequency and bandwidth of the transmission, the timing of the transmission and the duration of the transmission. U.S. Pat. No. 3,252,225 issued to Hixon and U.S. Pat. No. 4,314,365 issued to Petersen teach selection of transmission wave length based upon pipe characteristics such as the length of pipe sections and the overall length of the drill string. U.S. Pat. No. 4,390,975 issued to Shawhan suggests delaying successive acoustic data transmissions to allow reflections of earlier transmissions to dissipate. Similarly, U.S. Pat. No.5,050,132 issued to Duckworth discloses transmissions of acoustic data signals only during preselected short time intervals to avoid data distortion. U.S. Pat. No. 5,124,953 issued to Grosso discloses selecting a passband frequency for acoustic data transmission that best correlates a measured and a modeled xe2x80x9cpower spectral densityxe2x80x9d of the acoustic transmission. U.S. Pat. No. 5,148,408 issued to Matthews similarly suggests the testing and finding of an optimum frequency for acoustic data transmission which results in the most efficient reception of the acoustic data under the circumstances then present in the well. The Matthews patent suggested period testing of data transmission through the drill string during drilling operations, finding an optimum frequency for transmission based upon drill string conditions at the time of testing, and changing the acoustic data transmission frequency as needed. U.S. Pat. No. 4,562,559 issued to Sharp et al, proposes a phase-shifted transmission wave having a broader frequency spectrum to bridge gaps in the passbands. U.S. Pat. No. 5,128,901 issued to Drumheller proposes transmission of acoustic data conditioned to counteract interference caused by the drill string. Prior to transmission, each signal frequency is multiplied by a factor designed to enhance data transmission.
In many communications systems it is possible to model the communication channel before the system is placed in service, then to design a channel equalizer to compensate for the channel distortion. Unfortunately, in an oil well the acoustic transmission environment changes continuously, so it is impossible to design a fixed acoustic channel equalizer which is tailored to the oil well environment. Further complicating acoustic equalization is the complex acoustic environment in an oil well which often contains non-linearities which cannot be effectively modeled using linear filtering techniques.
From the foregoing, it is apparent that a need exists for improved methods of acoustic data transmission and, in particular, a need exists for utilizing such improved methods of acoustic data transmission in oil well environments. Furthermore, it would be desirable to provide such methods which compensate for changes in the environments in which the acoustic data transmission occurs.
The invention is directed toward a method and apparatus for data communication in an oil well environment. The method comprises detecting an acoustic signal transmitted along an acoustic channel, the acoustic signal being distorted from transmission through the acoustic channel, generating a transmitted data signal in response to the acoustic signal, inputting the transmitted data signal to an adaptive equalizer and adaptively equalizing the transmitted data signal to produce an equalized data signal related to the transmitted data signal by a mathematical function. The detecting step may include positioning an acoustic receiver in a communication unit along the acoustic channel and the acoustic receiver may comprise at least one accelerometer. The communication unit may be positioned downhole and the adaptive equalizer may be positioned remotely relative to the communication unit or may be placed in the communication unit. The transmitted data signal may be in digital format. The adaptive equalizer may be a frequency domain filter, a neural net adaptive equalizer or a nonlinear recurrent neural net equalizer.
The acoustic signal may comprise a plurality of discrete transmissions which may be in binary code. The plurality of discrete transmissions may include a training sequence for training the adaptive equalizer and may comprise a first discrete transmission transmitted repeatedly.
In another embodiment, the method of data communication in an oil well environment may comprise the steps of transmitting an acoustic signal from a first location along an acoustic channel, detecting the acoustic signal at a second location along the acoustic channel, the acoustic signal being distorted from transmission through the acoustic channel, generating a transmitted data signal in response to the acoustic signal, inputting the transmitted data signal to an adaptive equalizer and adaptively equalizing the transmitted data signal to produce an equalized data signal related to the transmitted data signal by a mathematical function. The transmitting step may further comprise positioning an acoustic transmitter in a first communication unit along the acoustic channel downhole or elsewhere. The acoustic transmitter may comprise a piezoelectric stack acoustic wave generator or other transmission apparatus. Detection may further comprise positioning an acoustic receiver in a second communication unit in the drill string. As before, the adaptive equalizer may be a neural net adaptive equalizer.
The method may further comprise acquiring data, generating an original data signal in response to the acquired data and inputting the original data signal to an acoustic transmitter. Acquisition may comprise positioning a data acquisition unit along the drill string. The data acquisition unit comprises at least one data transducer such as a temperature sensor in communication with a downhole temperature source. The data acquisition unit may comprise a terminal capable of receiving data input.
The acoustic signal may comprise a series of acoustic training signals for training the adaptive equalizer. The series of acoustic training signals may be transmitted at a predetermined time. A stored training signal may be placed in storage and may include a series of stored training data signals corresponding to the series of acoustic training signals. At least a portion of the stored training signals may be cross-correlated to the transmitted data signal. The acoustic signal may comprise a notification signal for notifying the adaptive equalizer of a training session and the acoustic signal may be repeatedly transmitted during well operations.